The present invention relates to a method of and an apparatus for controlling a direct coupling mechanism such as a lockup clutch or the like to mechanically connect and disconnect the input and output members of a hydrodynamic driving apparatus such as a torque converter or the like.
Conventional automatic transmissions for use in automobiles are generally in the form of a combination of a hydrodynamic driving apparatus such as a torque converter, for example, and a transmission gear shifting mechanism. The hydrodynamic driving apparatus suffers slippage during the transmission of engine power since the engine power is transmitted through a fluid in the hydrodynamic driving apparatus. The slippage thus caused results in poor fuel economy and an increase in the engine rotational speed which in turn produces greater engine sounds.
To avoid the above drawbacks, some transmissions employing such a hydrodynamic driving apparatus includes a direct coupling mechanism such as a lockup clutch, for example, for directly mechanically connecting the input and output members of the hydrodynamic driving apparatus (e.g. the impeller and turbine of a torque converter). The engine power is transmitted through the torque converter only while the automobile is running in a low speed range, or when gear shifts are effected, and the lockup clutch is engaged for improved fuel economy and reduced engine sounds in other occasions.
The lockup clutch may be controlled such that it is simply engaged or disengaged, or it is selectively engaged, partly engaged, and disengaged. The latter control is effected in a certain driving mode in low- and medium-speed ranges. According to this control process, the torque converter is not completely directly connected, but the lockup clutch or direct coupling mechanism is controlled to cause slippage when the torque varies at a peak value. For example, the ratio e of the rotational speeds of the input and output members of the torque converter, or a slip ratio (1-e), is calculated, and fed back for controlling the direct coupling mechanism so that the speed ratio e will not become 1 or the slip ratio will not become 0 in the aforesaid certain driving mode. Such a control method is disclosed in Japanese Laid-Open Patent Publication No. 61-286665, for example.
With the lockup clutch being thus variably engageable under the feedback control, however, a system for controlling the amount of engagement of the lockup clutch is inevitably subject to a certain delay in operation. In addition, the control process is adversely affected by the detecting errors of sensors which detect the rotational speeds of the input and output members of the torque converter or an error produced in calculating the speed ratio or slip ratio. For these reasons, the amount of engagement of the lockup clutch may be excessively or insufficiently corrected, and hence the lockup clutch may not stably controlled, with the results that the rotational speed of the input or output member of the torque converter tends to surge or vary.
In order to solve the above problems caused by the error in detecting the rotational speeds or the error in calculating the speed ratio or the slip ratio, the applicant has proposed a control method by which a control value for the amount of engagement of a lockup clutch is maintained at a constant level for a predetermined period of time, and a control value for a next period of time is determined based on the ratio e of the rotational speeds of the input and output members of a torque converter at the end of the predetermined period of time.
According to the proposed control method, the amount of engagement of the lockup clutch can be stably controlled to suppress any surging or variation of the rotational speed of the output member of the torque converter. If the control method is carried out while the automobile is being accelerated, for example, the speed ratio e increases even when a control value for the amount of engagement of the lockup clutch is constant. Therefore, where the speed ratio e is detected at the end of a predetermined period of time and a control value for a next period of time is determined based on the detected speed ratio e on a real-time basis, the amount of clutch engagement is overly corrected, lowering the engine rotational speed. As a result, the automobile runs in an embarrassing situation in which it is accelerated while the engine rotational speed is being lowered.
The direct coupling mechanism or lockup clutch is controlled by employing two control hydraulic pressures, i.e., a modulated pressure supplied by turning on and off two solenoid valves, and a throttle pressure commensurate with the opening of the throttle valve of the engine. These control hydraulic pressures are used to control the operation of a lockup shift valve, a lockup control valve, and a lockup timing valve for engaging and disengaging the lockup clutch. The throttle pressures is also employed to control hydraulic clutches for effecting gear shifts. Since the throttle hydraulic pressure fluctuates when a gear shift is made, therefore, the control of the lockup clutch is liable to become unstable.